1. Field of the Invention
The present invention relates to a process for bleaching substantially lignin-free pulp or paper using an aqueous bleaching solution containing hydroxymethane sulfinic acid (HAS). In particular, this invention relates to a process for bleaching substantially lignin-free pulp or paper using an aqueous bleaching solution containing HAS alone or with either additional reductive bleaches or with bleaching initiators or both.
2. Brief Description of Art
In the last decade or so, recycled mixed waste paper has been increasingly used as a source for printing and writing paper and other commonly used paper materials. In fact the mixed waste paper termed as xe2x80x9cmixed office wastexe2x80x9d or xe2x80x9coffice packxe2x80x9d was not used before the 1990""s and is one of the primary raw materials defined as substantially lignin-free pulp. Generally, this recycled mixed waste paper is substantially lignin-free and contains inks and dyes that need to be removed. Such waste papers have been treated with a reductive bleaching agent such as sodium hydrosulfite (also known as hydro) or formamidine sulfinic acid (also known as FAS) to both bleach the waste paper and color-strip out of the dyes. However, each of these bleaching agents have significant operating problems associated with them. Hydro and FAS have flammability or spontaneous combustion problems and require separate storage areas. They also will decompose readily and thus have a short shelf life and are difficult to store for long periods of time. Furthermore, hydro can be shipped in aqueous solutions that have concentrations of no greater than 15% by weight. It has to be used soon after being made or it will decompose. Thus, it is desirable to use hydro at locations close to where it is made so as to avoid excess shipping costs due to water and loss of product due to decomposition. FAS cannot be dissolved in water in concentrations greater than 3% in solution, which makes it commercially unacceptable. When using FAS as an unstable powder, it is detrimental since it is a health hazard due to residual thiourea. Since both hydro and FAS are safety concerns, unstable in water and they react quickly with air and other entrained reducible chemicals (i.e. metals), their use to bleach and color-strip mixed waste paper fibers is greatly hindered. They are commonly used in short stages because of their rapid reaction. But in many instances mills have no choice but to use High Density Storage towers which have several hours retention which causes problems in reversion of the pulp or paper product since the current commercial bleaches are consumed in minutes. Accordingly, there is a need to find an improved process for bleaching and color-stripping mixed recycled paper and pulp as well as other substantially lignin-free papers and pulp.
Separately, hydroxymethane sulfinic acid (also known as HAS) has been proposed as a reductive chemical for textiles and mechanical or ground wood pulp and papers.
Several references discuss these various potential uses for hydroxymethane sulfinic acid:
Japanese Patent No. 78029722 teaches first bleaching unbleached or partially bleached wood pulp or lignified mechanical pulp or used paper with a two-stage bleaching process of hydrogen peroxide and then Rongalite (Hydroxymethane Sulfinic Acid).
Soviet Union Published Patent Application No. 1414901 describes joint bleaching of pulp and wood pulp (i.e. mechanical fully lignified pulp) using sodium hydrosulfite or Rongalite.
Soviet Union Published Patent Application No. 1560663 describes bleaching of wood pulp (i.e. mechanical fully lignified pulp) using a mixture of trisodium phosphate, Rongalite and sodium hydrosulfite.
Canadian Patent Application No. 2,128,814 teaches adding a treating agent (e.g. sodium hydrosulfite, formamidine sulfinic acid (FAS) or sodium formaldehyde sulfoxylate to kraft pulp during a final stage to increase the strength of the final paper product.
U.S. Pat. No. 4,113,427 (Fono et al.) describes a process for dying cellulosic textiles with vat and sulfur dyes using a reducing agent composition that contains 85-99% by weight sodium dithionite (also known as sodium hydrosulfite or hydro) and 1-15% sodium hydroxymethane sulfinate.
U.S. Pat. No. 4,166,717 (Fono et al.) describes a process for dying cellulosic textiles with indigo dye which includes first aging the dye solution for about 12 hours; then adding sodium dithionite and sodium hydroxide to the aged dye solution; then incorporating an aldehyde sulfoxylate addition product to stabilize the aged dye solution; and then immerging a cellulosic textile in the stabilized dye solution.
U.S. Pat. No. 4,676,961 (Appl et al.) teaches a stabilized water-containing solution dithionite formulation that contains certain amounts of sodium and/or potassium salts (e.g. sodium hydroxy methane sulfinate).
None of these references teach or suggest using an aqueous bleaching solution that contains HAS in the bleaching stage to bleach a substantially lignin-free pulp or paper.
Therefore, one aspect of the present invention is directed to a process for bleaching recycled substantially lignin-free pulp or paper in either a single-stage bleaching process or a multi-stage bleaching process; comprising
contacting said substantially lignin-free pulp or paper with an aqueous reductive
bleaching solution comprising hydroxymethane sulfinic acid during a reductive
bleaching stage for sufficient amount of time to at least partially bleach (and sometimes
preferably fully bleached) said substantially lignin-free pulp or paper.
Another aspect of the present invention is directed to an aqueous reductive bleach solution comprising hydroxymethane sulfinic acid with at least one bleaching initiator.
It has been found that the use of HAS alone or in combination with one or more reductive bleaches such as hydro or FAS gives superior brightness over those other reductive bleaches used alone. Furthermore, HAS has the advantages that it will not rapidly decompose or ignite when used in wet or in extreme ambient conditions (e.g. a hot paper mill environment). HAS does not need to be stored in a separate section of the paper mill because it is not self-igniting or a flammable hazardous material. HAS can be stored as a powder or liquid for months without losing strength. HAS can be made into an aqueous solution having concentrations up to about 40%, thus making it better suited for commercial transportation and use. HAS will work better than hydro or FAS in processes that require long bleaching steps because it does not react as quickly with oxidants such as entrained air or decompose as rapidly in water. Also, it can increase the maximum bleach response or ceiling brightness while stabilizing the system when combined with other reductive bleaches. It can be used with a reductive initiator to obtain a further increased bleach response since it is more stable than hydro or FAS.